1. Field of the Invention
The present invention relates to a multilayer ceramic substrate, and more particularly, to a multilayer ceramic substrate that has improved strength and is produced in accordance with a non-shrinkage process.
2. Description of the Related Art
Japanese Patent No. 3601671 describes a method for manufacturing a composite laminate, in which a first sheet layer including a first powder and a second sheet layer including a second powder which is not sintered at a sintering temperature for the first powder are alternately stacked, and in a firing step, the first powder is sintered, whereas the material of the first sheet layer is allowed to flow into the second sheet layer to fix the second powder. This manufacturing method has been attracting attention as a technique for manufacturing a multilayer ceramic substrate, which can prevent shrinkage in the direction of the principal surface.
When using the manufacturing method described above, the second sheet layer must be relatively thick so as to cause a constraining force to act sufficiently to prevent the first sheet layer from being shrunk, and at the same time, has to be thin enough to cause the material of the first sheet layer to flow for the densification.
In this regard, the applicant of the present application has proposed in Japanese Patent Application Laid-Open No. 2002-94244 that a constrained layer (the second sheet layer) allowed to contain a softened fluid powder (for example, glass) enables the constrained layer to be densified even when the constrained layer is thick. In this case, as the softened fluid powder, a powder is used which fails to soften or flow substantially at the shrinkage onset temperature of a ceramic raw material powder (the first powder) in a base material layer (the first sheet layer), but can soften or flow for the densification of the constrained layer at the completion of sintering.
However, even in the case of using the technique described in Japanese Patent Application Laid-Open No. 2002-94244, the constrained layer is formed such that it is thinner than the base material layer as in the case of the technique described in Japanese Patent No. 3601671.
As the ceramic raw material powder for use in the base material layer described above, a variety of glass ceramics can be used such as a glass-based low-temperature sintering ceramic material of an alumina powder mixed with borosilicate glass powder. However, because of the expensiveness of the glass powder, in the case of giving more consideration in terms of cost, for example, a non-glass based low-temperature sintering ceramic material is used which produces a glass component during the firing of a Ba—Al—Si based oxide ceramic or the like.
On the other hand, in the case of using a Ba—Al—Si based oxide ceramic the base material layer, an alumina powder mixed with a Ba—Al—Si based glass powder is used as a material for the constrained layer, because it is necessary to satisfy the requirement for the softened fluid powder described above (see, for example, Japanese Patent Application Laid-Open No. 2009-170566). It is to be noted that Japanese Patent Application Laid-Open No. 2009-170566 also discloses a structure in which a constrained layer is formed to be thinner than a base material layer.
However, it has been found that it is not possible to achieve a desirable substrate strength, when the materials as disclosed in Japanese Patent Application Laid-Open No. 2009-170566, specifically, the Ba—Al—Si based oxide ceramic, is used as the base material layer whereas the alumina powder and the Ba—Al—Si based glass powder are used as the constrained layer in the multilayer ceramic substrate for which the constrained layer is formed to be thinner than the base material layer, as described in Japanese Patent No. 3601671, Japanese Patent Application Laid-Open No. 2002-94244, and Japanese Patent Application Laid-Open No. 2009-170566.